Sunday, 19 October 2014

Weekend Plot: Bs mixing phase update

Today's featured plot was released last week by the LHCb collaboration:

It shows the CP violating phase in Bs meson mixing, denoted as φs, versus the difference of the decay widths between the two Bs meson eigenstates. The interest in φs comes from the fact that it's one of the precious observables that 1) is allowed by the symmetries of the Standard Model, 2) is severely suppressed due to the CKM structure of flavor violation in the Standard Model. Such observables are a great place to look for new physics (other observables in this family include Bs/Bd→μμ, K→πνν, ...). New particles, even too heavy to be produced directly at the LHC, could produce measurable contributions to φs as long as they don't respect the Standard Model flavor structure. For example, a new force carrier with a mass as large as 100-1000 TeV and order 1 flavor- and CP-violating coupling to b and s quarks would be visible given the current experimental precision. Similarly, loops of supersymmetric particles with 10 TeV masses could show up, again if the flavor structure in the superpartner sector is not aligned with that in the Standard Model.

The phase φs can be measured in certain decays of neutral Bs mesons where the process involves an interference of direct decays and decays through oscillation into the anti-Bs meson. Several years ago measurements at Tevatron's D0 and CDF experiments suggested a large new physics contribution. The mild excess has gone away since, like many other such hints. The latest value quoted by LHCb is φs = - 0.010 ± 0.040, which combines earlier measurements of the Bs → J/ψ π+ π- and Bs → Ds+ Ds- decays with the brand new measurement of the Bs → J/ψ K+ K- decay. The experimental precision is already comparable to the Standard Model prediction of φs = - 0.036. Further progress is still possible, as the Standard Model prediction can be computed to a few percent accuracy. But the room for new physics here is getting tighter and tighter.

9 comments:

Anonymous
said...

Maybe a silly question but the effect of new physics at scale M, must decouple right -- like v_weak/M or maybe (v_w/M)^2. So if M ~ 100 or 1000 TeV like you mention isn't this a small factor ~ 0.001-0.0001 (or maybe even 0.000001-0.00000001 if its (v_w/M)^2)?

That's right, new physics decouples like (v/M)^2. But the standard model contribution is so small (due to the 1-loop and CKM suppression) that even such a small effect can be visible - that's the beauty of flavor physics.

We all know model builders can usually find a way to hide new physics. So question. Assume there is no new physics discovered at the LHC, and in addition there was sufficient time to do a large amount of precision physics and analysis (say 20+ years).

Would a null result shrink the 'plausible' parameter space of new physics to such a small subset to remove the case to build a larger hadron collider (say with an order of magnitude larger reach than the LHC) or would there still be a large viable model building corner that would be left untested?

Yes, there will always be corners that can be accessed only by direct production in high-energy colliders. Now, how much weight you give to these corners is a question of taste. For me, personally, the current precision measurements imply that theories predicting many new degrees of freedom below 10 TeV (like SUSY, extra dimensions, or composite Higgs) are strongly disfavored.

Patience, patience,Patience in the zeptospace,Each null resultis the chance of a new symmetry!The glad surprise may come,A signal, not a statistical fluke,The faintest track,A missing energycould bring hopeWhen LHC will run again (loosely based on the penultimate stanza of "Charmes" from the French poet Paul Valéry http://www.lesarbres.fr/index.php?page=texte-valery2.php)

Hi Jester, a quick correction: This plot doesn't contain just the Jpsi K+K- and Jpsi pi+ pi- results from LHCb- there's a new result using Bs->Ds+Ds- included in there as well. It's much less sensitive, but potentially quite interesting as it is the first time phi_s has been measured in a final state without muons: http://arxiv.org/abs/1409.4619

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